1. Field of the Invention
This invention relates to the art of aluminium alloys and more particularly, to the production of superplastic high strength Al--Zn--Mg or Al--Cu alloy sheets.
2. Description of the Prior Art
By the term "superplasticity" is meant a phenomenon where a material is able to be elongated to an extent of several hundreds to one thousand percent under certain external conditions without necking of the material. The superplasticity may be broadly divided into two categories including transformational superplasticity utilizing phase transformation by thermal cycling and fine grain superplasticity (structural superplasticity) which will be observed in fine grain crystal materials. In order to cause the fine grain superplasticity, it is essential that the crystal grain size of material be controlled to be fine.
In general, high strength aluminium alloys are produced by a sequence of steps including homogenizing a cast ingot at a temperature of 400.degree. to 550.degree. C., subjecting the homogenized ingot to hot and cold workings at a temperature of 350.degree. to 550.degree. C., followed by subjecting to solution treatment at a temperature of 450.degree. to 550.degree. C. and aging. The alloys obtained by the above ordinary steps have a grain size as large as 40 to 100 .mu.m. Even when the alloys are deformed at high temperatures, superplasticity cannot be obtained.
In order to obtain high strength Al--Zn--Mg or Al--Cu alloys, several methods have been heretofore proposed, typical of which is described in U.S. Pat. No. 4,358,324. In the thermomechanical processing method, high strength aluminium alloys such as 7475, 2219 and the like are treated by dissolving some of precipitating constituents in the alloy by heating the alloy to a solid solution temperature, overaging and recrystallizing the alloy by warm working and quick heating. The warm working must be effected at a high rolling reduction and cannot thus be applied to thick sheets. Additionally, when intermetallic compounds of solute elements precipitate in excess due to the overaging treatment, little effects of fine graining are produced.